In image display devices having a backlight such as liquid crystal display devices, by controlling the luminance of the backlight based on an input image, the power consumption of the backlight can be suppressed and the image quality of a displayed image can be improved. In particular, by dividing a screen into a plurality of areas and controlling, based on an input image in an area, the luminance of backlight light sources provided in the area, a further reduction in power consumption and a further improvement in image quality can be achieved. A method of driving a display panel while thus controlling the luminance of backlight light sources based on an input image in each area is hereinafter referred to as “area active drive”.
A liquid crystal display device that performs area active drive uses, for example, LEDs (Light Emitting Diodes) of three RGB colors or white LEDs, as backlight light sources. The luminances of LEDs provided in each area are determined based on the highest value and mean value of the luminances of pixels in the area, etc. The determined luminances are provided to a backlight drive circuit as LED data. In addition, based on the LED data and an input image, display data (data for controlling the light transmittances of liquid crystals) is generated, and the display data is provided to a liquid crystal panel drive circuit. Note that the luminance of each pixel on a screen is the product of the luminance of light from a backlight and a light transmittance based on display data. Here, light emitted from a single LED hits a plurality of areas around a corresponding area. Thus, the luminance of each pixel is the product of the sum of the luminances of lights emitted from a plurality of LEDs and a light transmittance based on display data.
According to a liquid crystal display device such as that described above, suitable display data and LED data are obtained based on an input image, and the light transmittances of liquid crystals are controlled based on the display data, and the luminances of LEDs provided in respective areas are controlled based on the LED data, whereby the input image can be displayed on the liquid crystal panel. When the luminance of pixels in an area is low, by reducing the luminance of LEDs provided in the area, the power consumption of the backlight can be reduced.
Note that in relation to inventions pertaining to this subject the following prior art documents are known. Japanese Patent Application Laid-Open No. 2005-338857 discloses an invention of a liquid crystal display device that has a backlight unit including a plurality of LEDs, as a direct-type backlight. In the invention, by controlling the luminances of the LEDs according to peak gray level values in respective divided regions of a liquid crystal display panel, an improvement in image quality and a reduction in power consumption are achieved. Japanese Patent Application Laid-Open No. 2005-234134 discloses an invention of a liquid crystal display device that includes, as light sources, a white light source that emits lights of three wavelengths or more and an auxiliary light source using LEDs. The liquid crystal display device achieves widening of a color reproduction range by optimizing the wavelength selection characteristics of a wavelength selection filter. Japanese Patent Application Laid-Open No. 2006-343716 discloses an invention of a liquid crystal display device in which color reproduction capability is enhanced by switching between LEDs that radiate white light and LEDs of three RGB colors, according to the brightness around a liquid crystal panel. Japanese Patent Application Laid-Open No. 2005-17324 discloses an invention of a liquid crystal display device that adjusts white balance by controlling the amounts of light from LEDs of three RGB colors independently of one another.